Heat-curable silicone elastomer compositions which cure through hydrosilylation reaction have been utilized as potting agents, coating agents, adhesives, and liquid injection molding compounds. They have the advantages of a very short curing time and no reaction by-products. However, since they are unstable during storage at room temperature, their components must be divided into two parts during storage, which are mixed on use, leaving a problem of cumbersome operation.
As a solution to this problem, it was recently proposed to add hydrosilylation reaction control agents to silicone elastomer compositions. Exemplary control agents are nitrile compounds, carboxylates, metal compounds such as stannous and mercuric compounds, sulfur compounds, benzotriazoles, acetylene compounds, hydroperoxides, and phosphorus compounds. With these control agents added, silicone elastomer compositions have good long-term shelf stability, but become less curable and take a longer time to complete curing.
Another approach to bypass this problem is to physically embed the hydrosilylation reaction catalyst in a material which is solid near room temperature, but liquid at elevated temperatures. For example, Japanese Patent Publication (JP-B) No. 41707/1978 discloses to add to a silicone elastomer composition a material having a platinum catalyst embedded in a silicone resin which will soften at the curing temperature. This approach can improve the shelf stability of the composition to some extent, but suffers from poor curing because the platinum catalyst embedded in the silicone resin is not quickly released in a short time and can be leached out with the lapse of time.
EP 347859A corresponding to JP-A 4833/1990 discloses microcapsulation of a platinum catalyst with a silicone resin. The platinum catalyst can be leached out to the composition with the lapse of time because of low compatibility. This method is thus less effective for extending the pot life and costly. U.S. Pat No. 4,784,879 corresponding to JP-A 47442/1989 discloses microcapsulation of a platinum catalyst with various organic polymers. The microcapsules are washed with a solvent in which the platinum catalyst is soluble, but not the organic polymer. This method, however, is complex and costly.
Therefore, there is a desire to have a technique capable of improving the shelf stability and curing properties of a heat-curable silicone elastomer composition in an industrially acceptable manner.